The closest known processes to that of this invention are described in Koslow U.S. Pat. Nos. 5,019,311; 5,147,722; 5,189,092; 5,249,948; and 5,331,037, their parent applications, their corresponding foreign patent applications and patents, and the references cited therein.
The above-mentioned patents disclose processes for the production of composite materials which are characterized by primary particles interconnected by a binder material. Some of these processes require high pressure and shear or extrusion through a die with carefully controlled back pressure. These prior art processes are extremely useful in producing a wide variety of articles including extruded solid forms such as activated carbon filters.
It would often be desirable to impregnate, cover, or otherwise treat a relatively fragile web base material with an active component such as a powdered adsorbent or absorbent material. One example would be a nonwoven medium coated with agents having water absorption and odor adsorption characteristics as in a diaper or hygiene products. A number of other related products will be apparent to those skilled in the art such as, for example, coated paper tissues and toweling, and fabrics such as surgical bandages and sanitary napkins. However, the fragile nature of the underlying base material would make it impractical to employ the known prior art techniques which require high pressure and shear.
In the prior art referred to above, the powdered active material is formed into a self-supporting structure by fusion of a thermoplastic material with which it is intimately mixed. However; the pressures, temperatures, and shear involved, or the process equipment used would not permit their application to fragile substrates such as the webs described herein. Accordingly, it is a primary object of the present invention to provide a method for continuously coating a relatively fragile web with a dry mixture of at least one particulate active material and a very finely divided particulate thermoplastic binder.
Another problem associated with using powdered active material is that the porosity does not adequately filter out bacteria and the like. Accordingly, it is an object of the present invention to provide a microporous properties to the treated web by using high porosity particulates having a particle size of between about 5-30 microns, that have an open packing characterization, whereby the coated web exhibits microporous filtration properties. Also, the present invention can include chemical treatment of either the active particles, binder particles or web itself in order to control or kill bacteria.
Other objects, features, and advantages will become apparent from the following description and appended claims.
In accordance with the present invention a loose, dry composite powder is formed which comprises at least one group of particles of an active ingredient and particles of a thermoplastic binder. The particle size of both the binder particles and the active ingredient, e.g., diatomaceous earth or perlite, is preferably in the range between about 5 to 30 microns so has to impart microporous filtration properties to the web upon which they are coated. The small size of the particles typically cause a mean flow path (MFP) of between about 4 to 6 microns when the mean particle size of the particulates is, for example, 16 microns. Thus, when a web coated with such small particles is spiral wound having, for example, 12 layers of such a coated web, a MFP of 0.7 microns can be achieved. Using these reduced size particulates as the coating of the web converts a relatively inexpensive open-porosity nonwoven web into a high performance material used capable of microfiltration. The active and binder particles, or optionally, the web itself, may be treated with a chemical coating which is capable of imparting properties which are capable of controlling or killing bacteria. The chemical treatment may include quaternary amines, milk protein, triclosan, silver impregnated zeolite or activated carbon. Such factory made precoats provide more uniformity in manufacture, greater flexibility, reduced cracking, pleatability and substantially reduced start-up costs. This chemical treatment provides micro-biological interception using protein solutions which may be applied to the particulates that would then be applied by wet or dry means to a substrate.
The mixture of active and binder powders is applied to the surface of a moving web by means of a knurled roller. The coated web, which can be preheated through a convective or infra-red oven, is then passed through the nip of a pair of rollers, one of which is heated, which apply both heat and pressure to fuse the thermoplastic binder to the active particles and to the underlying web. This step may also be employed to incorporate a second web to achieve a sandwich effect with the active material incorporated between two web surfaces. Upon leaving the heated rollers, the thermoplastic binder sets to form a single, composite structure.